It is known that, for example in nuclear reactors, the cover is detachably fastened on the vessel in order to enable the reactor to be periodically recharged with fuel and to take the opportunity to inspect the interior of the vessel and the components situated in it.
For this purpose the cover is fastened on the reactor vessel by studs, which are screwed into the flange of the vessel and which pass through plain holes provided in the flange of the cover. A fairly large number of these studs, disposed relatively close to one another, are generally used, so that the removal and refitting of the cover require the unscrewing and screwing in of all these studs, in addition to their tensioning.
These operations of screwing the studs in and out are uncertain and delicate. Each stud in fact weighs several hundred kilograms, so that during the screwing of the stud into or out of the threaded hole it is necessary to overcome frictional resistance which occurs in the screw thread and which with the weight of the stud. Furthermore, the axes of the different components of the screwing apparatus do not always coincide with the axes of the threaded holes in the vessel, or with the axes of the studs, after a period of operation. Consequently, it frequently occurs that coaxiality of the stud cannot always be achieved automatically when it is introduced into the threaded hole in the vessel, and this operation, as well as the first few turns for the screwing-in of the stud, must be carried out manually. This has the consequence that the operators carrying out this work must remain for too long a time in a zone of excessively high radioactivity.
These alignment faults, which may also give rise to the jamming of the stud while it is being screwed into the receiving threaded hole in the vessel, subject the moving parts to stresses distributed irregularly around their axis of translation or of rotation, which, as the result of friction, may give rise to irregular premature wear, and also to the formation of "tinsel", i.e., long shavings of small section formed from the metal of the screw thread, which is softer than the stud, through excessive friction between the stud and the receiving screw thread.